1. Field of the Invention
The present invention relates to aircraft fuel pipe couplings and in particular but not exclusively flexible pipe couplings for connecting a double-walled pipe in an aircraft fuel system to another fuel pipe.
2. Discussion of Prior Art
In aircraft fuel systems it is often necessary to use double-walled pipes, connectors and couplings in order to protect from fuel leakage. When the risk of leakage is high, for example in the pressurised fuselage of an aircraft, the entire fuel system may be double-walled. Where the risk of leakage is less, it may only be the couplings that are double-walled. Further complications arise from the need to use flexible couplings in order to compensate for manufacturing tolerances, structural deflections or thermal effects.
FIG. 1 of the attached drawings shows a prior art coupling connecting a single-walled pipe (not shown, but positioned to the left of the coupling as shown in FIG. 1) and a double-walled pipe 105 (to the right of the coupling as shown in FIG. 1). The coupling comprises (a) a rigid coupling part 101 and (b) a flexible coupling part 108. The rigid coupling part 101 is rigidly attached to a single-walled pipe by means of a connector 106. The flexible coupling part 108 comprises an inner flexible coupling 103, an outer flexible coupling 104 joined to the double-walled pipe 105 and an outer removable sleeve 102. The double-walled pipe 105 and connector 106 are connected to each other via the inner flexible coupling 103 and are allowed to float relative to each other by means of two O-ring seals 109 of the inner flexible coupling 103. The outer flexible coupling 104 similarly comprises one O-ring seal 110 allowing axial movement between the outer flexible coupling 104 and the outer removable sleeve 102. The movement provided by the seals 109, 110 of the inner and outer flexible couplings 103, 104 have different centres of rotation. This can subject the coupling to side loads that increase seal and coupling wear and can limit the angular deflection allowable between the opposite ends of the coupling, which may for example rotate relative to each other about axis 107. The positioning of the seals 109, 110 also makes the coupling relatively bulky, thereby requiring a relatively large volume in which to fit. The coupling includes a large number of components that create complexity in the manufacturing process. The prior art coupling also requires complex electrical bonding arrangements between the pipes.